A typical beverage dispensing system includes a container for the beverage or beverage component, a dispensing head, a conduit leading from the container to the dispensing head and a pump for pumping liquid from the container to the dispensing head. The container may contain the beverage, such as wine, which is to be dispensed. Alternatively, the container may contain a beverage component, such as a soft drink syrup, a tea syrup or juice concentrate. In this latter case, a second portion of the system supplies water, which may be carbonated, to the dispensing head which mixes the drink components just prior to dispensing the beverage.
A common form of container for a beverage dispensing system is a collapsible plastic bag contained within a box. In order to pump the bag dry and to have a relatively high output pressure, it is normally preferred to use a fixed displacement pump. Although a centrifugal pump could be used, generally they are not economically feasible for this purpose.
One problem with a fixed displacement pump is that it has a fixed discharge rate for a given speed of operation. Thus, the ideal fixed displacement pump has a pump curve that would provide a constant discharge rate for all operating pressures. In practice, the pump curve has some slope but still does not have a wide range of flow rates. Unfortunately, in a beverage dispensing system, the pump may be called upon to deliver a wide range of flow rates servicing one or more dispensing needs simultaneously and be subjected to numerous variable conditions, such as different viscosity liquids and different lengths and diameters of discharge lines leading from the pump.
Although the stroke length or speed of the pump could be changed, the former reduces the self-priming ability of the pump and the latter is expensive. It is known to alter the shape of a pump curve by bypassing liquid from the discharge side of the pump. However, if this is done, it is not possible to control the pump with a pressure switch which is responsive to the discharge pressure of the liquid being pumped.